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Manufacturers size anchors by boat length and displacement ranges. A 30-foot cruiser typically needs a 15–22 lb (7–10 kg) modern anchor or equivalent holding power.
Anchor class = manufacturer chart (length + displacement)Scope is rode length divided by water depth (including bow height). Minimum 5:1 for chain, 7:1 for all-rope in calm; 10:1 for overnight or storm conditions.
Rode length = (depth + bow height) × scope ratioWind force on boat increases with wind speed squared. Larger displacement boats need heavier anchors and longer scope for the same wind speed.
Force ∝ wind speed² × exposed lateral areaUpdated: July 2026
10 ft depth + 4 ft bow height = 14 ft. At 7:1 scope: 98 ft rode minimum. Use 100 ft chain-rope combo; 15 kg Bruce or 14 kg Mantus for 30–35 ft range.
8 ft depth, 5:1 scope on chain: 40 ft rode sufficient for 15-knot max wind. Lighter lunch anchor acceptable if crew remains aboard.
Increase scope to 10:1: 140 ft rode at 14 ft depth. Verify anchor one size up from standard recommendation; deploy second anchor if exposed anchorage.
Gusts to 30 knots pull horizontally on the rode. Short scope lifts the anchor shank and breaks holding. Deploy at least 7:1 for overnight; 10:1 in exposed locations.
Strong rode with weak anchor still drags. The anchor must hold the boat; rode only transmits load. Upgrade anchor before upgrading rope diameter.
Proper ground tackle keeps your boat secure in changing conditions. This calculator recommends anchor weight class and minimum rode length from boat length, displacement, and expected wind conditions.